High-aperture-ratio inner-focus telephoto lens

ABSTRACT

A high-aperture-ratio inner-focus telephoto lens comprises a first lens group of positive refracting power, a second lens group of negative refracting power and a third lens group of positive refracting power arranged in the named order from the object side, in which the first lens group and the second lens group form a substantially afocal system, focusing is effected by moving the second lens group along the optical axis, and the first lens group has a front group of positive refracting power and a rear group of weak positive refracting power arranged in the named order from the object side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-aperture-ratio inner-focus telephoto lens used for example in single-lens reflex cameras and electronic still cameras, and more particularly to a high-aperture-ratio inner-focus telephoto lens having an angle of view equivalent to that of a lens with focal length of about 200 mm in case of 35 mm camera.

2. Related Background Art

A conventional high-aperture-ratio inner-focus telephoto lens is one having an angle of view equivalent to that of a lens with focal length of about 200 mm in case of 35 mm camera as disclosed for example in Japanese Laid-open Patent Application No. 53-134425, No. 59-116709 or No. 1-102413.

The lens as disclosed in Japanese Laid-open Patent Application No. 53-134425 has a big focusing lens group, which, in autofocus, forces a great load on a drive motor for the focusing group. Thus, the lens was disadvantageous in autofocus. Also, the lens as disclosed in Japanese Application No. 59-116709 has a focusing lens group composed of two groups which are moved at mutually different speeds, enabling compensation of aberration change in the near range. It had, however, drawbacks of complication and size increase of mechanism, because the focusing group was composed of the two groups. The lens as disclosed in Japanese Application No. 1-102413 has a nearest object distance of 2.5 m, which is relatively long among telephoto lenses having an angle of view equivalent to that of the lens with focal length of about 200 mm in case of 35 mm camera. Further, the imaging performance of the lens was not always satisfactory for shooting in the near range.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a high-aperture-ratio inner-focus telephoto lens which is excellent in optical performance, simple in focusing mechanism, compact in size, and small in aberration change in shooting in the near range.

In an aspect of the present invention, achieving the above object, a high-aperture-ratio inner-focus telephoto lens comprises a first lens group G₁ of positive refracting power, a second lens group G₂ of negative refracting power and a third lens group G₃ of positive refracting power arranged in the named order from the object side, wherein said first lens group G₁ and said second lens group G₂ form a substantially afocal system, focusing is effected by moving said second lens group G₂ along the optical axis, and said first lens group G₁ has a front group G₁₁ of positive refracting power and a rear group G₁₂ of weak positive refracting power arranged in the named order from the object side.

In the above arrangement, the high-aperture-ratio inner-focus telephoto lens according to the present invention is composed of the three lens groups, which are positive, negative and positive in power, arranged in the named order from the object side. The first lens group G₁ of positive power and the second lens group G₂ of negative power are arranged such that the object focal point of second lens group G₂ is made coincident with the substantial image focal point of first lens group G₁.

Then, the second lens group G₂ is moved for focusing such that the object focal point of second lens group G₂ is always kept near the position of focal point of first lens group G₁.

Consequently, rays entering the third lens group G₃ are always kept substantially parallel to the optical axis, and the parallel rays form an image on a film plane through the third lens group G₃.

In the present invention, the refracting power of first lens group G₁ is constituted by a combination of the front group G₁₁ and the rear group G₁₂, which is advantageous in aberration compensation.

Such an arrangement can provide a high-aperture-ratio inner-focus telephoto lens which is excellent in optical performance, simple in focusing mechanism, compact in size, and small in aberration change in shooting in the near range.

The size of such photographic optical system can be decreased by shortening the focal length f₁ of first lens group G₁. Shortening the focal length f₁ of first lens group G₁ can decrease a gap between the first lens group G₁ and the second lens group G₂, which allows the size reduction of the total photographic optical system.

Also, by decreasing a movement amount of the second lens group G₂ which is the focusing group, a gap can be decreased between the second lens group G₂ and the third lens group G₃ while keeping the nearest object distance relatively short, which enables further size reduction of the total optical system.

Here, in order to keep small the movement amount of the second lens group G₂ as the focusing group, it is necessary to keep small for a movement amount of object point a movement amount of an image formed by the first lens group G₁ having the focal length f₁.

Considering the first lens group G₁ as thin lens, the following relation stands from the lens equation of image formation, where a is an object point distance and b is an image point distance.

    1/a+1/b=1/f.sub.1 →f.sub.1 =a/(a/b+1)               (A)

Letting α be a longitudinal magnification, the longitudinal magnification α can be expressed by the following equation. ##EQU1## It is seen from this equation that if the object point moves from an arbitrary position and the object point is again fixed relative to the first lens group G₁, that is, if the object point distance is set as a=constant, the movement amount of image point by the first lens group G₁ for the movement amount of object point can be reduced by making the longitudinal magnification α smaller.

Substituting Equation (B) into Equation (A), ##EQU2## Then, decreasing the longitudinal magnification α also decreases the focal length f₁.

Thus, shortening the focal length f₁ of first lens group G₁ can decrease the movement amount of second lens group G₂ in focusing for a same near-range object. Also, the gap can be decreased between the second lens group G₂ and the third lens group G₃ and the gap can be also decreased between the first lens group G₁ and the second lens group G₂, whereby the total optical system can be constructed in reduced size.

However, if the first lens group G₁ has a too strong refracting power, the first lens group G₁ itself causes great spherical aberration, resulting in degrading the aberration of the total photographic optical system. Then, the following condition is preferably satisfied in order to obtain acceptable spherical aberration with less focusing movement amount.

    0.60<φ/f.sub.1 <0.77                                   (1)

The condition of Equation (1) defines a ratio between focal length f₁ of first lens group G₁ and effective aperture φ of an object-side lens surface of a positive lens element closest to the object in the first lens group. Above the upper limit of the condition of Equation (1), the focal length f₁ of the first lens group G₁ is too short relative to the effective aperture φ, which would result in increasing the spherical aberration of the first lens group G₁ itself so as to be unable to be compensated. Further, the first lens group G₁ in the present invention is composed of the front group G₁₁ of positive refracting power and the rear group G₁₂ of weak positive refracting power arranged in the named order from the object side, but in the range above the upper limit the front group G₁₁ cannot be constructed of such a small number of lens elements as in embodiments of the present invention. In addition, the secondary chromatic spherical aberration also increases so as to be unable to be compensated.

Conversely, below the lower limit of the condition of Equation (1), the focal length of first lens Group G₁ becomes longer, which increases the focusing movement amount undesirably.

In the present invention, if the first lens group G₁ has a strong refracting power, the refracting power of the first lens group G₁ is properly assigned to the front group G₁₁ and the rear group G₁₂, so that the aberration compensation can be advantageously effected in the first lens group G₁. Also, the front group G₁₁ converges rays emergent from an object point on the optical axis (as will be referred to as Rand rays) to make them enter the rear Group G₁₂.

If the air gap is set smaller between the front group G₁₁ and the rear Group G₁₂ in such an arrangement, the Rand rays enter the rear Group G₁₂ at positions far from the optical axis, which is advantageous in compensation of spherical aberration in the rear Group G₁₂. However, if the air gap is set smaller between the front Group G₁₁ and the rear Group G₁₂, the outer diameter of rear Group G₁₂ increases, which is disadvantageous in size reduction and weight reduction of photographic optical system.

Then, the following condition of Equation (2) is preferably satisfied in order to compensate the spherical aberration satisfactorily while keeping small the outer diameter of rear group G₁₂ in the first lens group G₁.

    0.10<D.sub.1 /Bf.sub.11 <0.26                              (2)

The condition of Equation (2) defines the air gap D₁ between the front group G₁₁ and the rear group G₁₂ with respect to back focus Bf₁₁ of front group G₁₁ in the first lens group G₁.

Above the upper limit of the condition of Equation (2), the air gap D₁ increases between the front group G₁₁ and the rear group G₁₂, so that the Rand rays enter the rear group G₁₂ at positions near the optical axis, which undesirably makes difficult the compensation of spherical aberration in the rear group G₁₂.

Conversely, below the lower limit of the condition of Equation (2), the air gap D₁ decreases between the front group G₁₁ and the rear group G₁₂, so that the Rand rays enter the rear group G₁₂ at positions too far from the optical axis, which results in increasing the outer diameter of the rear group G₁₂ unpreferably.

Specifically speaking of the rear group G₁₂ in the first lens group, the rear group G₁₂ is preferably composed of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side arranged in the named order from the object side in order to obtain better imaging performance. Since the rear group G₁₂ is a lens group for auxiliarily compensating the spherical aberration, it is preferably arranged in the meniscus form with a convex surface on the object side to avoid influence on rays except for the Rand rays.

Although the rear group G₁₂ will be described as a lens group of bonded elements for simplicity of lens barrel structure in each embodiment as will be described, it may be constructed as a lens group of two separate lens elements in order to increase the degree of freedom in aberration compensation.

In the present invention, the first lens group G₁ and the second lens group G₂ form a substantially afocal system, so that the third lens group G₃ has brightness equivalent to that of the total optical system. Therefore, as the optical system increases its aperture as in the present invention, the load increases on the aberration compensation in the third lens group G₃ in such arrangement. Then, the third lens group G₃ is preferably constructed as follows in order to reduce the size of the third lens group G₃ and to improve the performance in aberration compensation.

The third lens group G₃ has a front group G₃₁ of weak positive refracting power and a rear group G₃₂ of positive refracting power. The front group G₃₁ is composed at least of a positive lens and a negative lens. The front group G₃₁ is effective to compensate the chromatic aberration and the spherical aberration, and the rear group G₃₂ has a stronger refracting power than that of the front group G₃₁ to attain most of the refracting power of the third lens group G₃. The arrangement of refracting power between the front group G₃₁ and the rear group G₃₂ is preferably set within the range defined by the following condition to effect proper aberration compensation.

    0.10<f.sub.3 /f.sub.31 <0.35                               (3)

The condition of Equation (3) defines a ratio of focal length f₃ of third lens group G₃ to focal length f₃₁ of front group G₃₁ in the third lens group G₃. Above the upper limit of the condition of Equation (3), the front group G₃₁ in the third lens group G₃ has a stronger refracting power, so that a load increases on the compensation of spherical aberration in the front group G₃₁. Thus, the compensation of spherical aberration could be insufficient. Further, it is unpreferable in respect of increase in change of upper coma with a change in object distance from infinity shooting to near range Shooting. Conversely, below the lower limit of the condition of Equation (3), the rear group G₃₂ in the third lens group G₃ has a too strong refracting power, which makes difficult the compensation of spherical aberration and coma by the front group G₃₁.

In such arrangement, the rear group G₃₂ in the third lens group preferably satisfies the following condition of Equation (4).

    -0.17<q.sub.32 <0.35                                       (4)

where q₃₂ is a dimensional factor of the rear group G₃₂ in the third lens group G₃ and q₃₂ =(R₂ +R₁)/(R₂ -R₁) where

R₁ : a radius of curvature of a surface closest to an object in the rear group G₃₂ in the third lens group G₃ ;

R₂ : a radius of curvature of a surface closest to an image in the rear group G₃₂ in the third lens group G₃.

The condition of Equation (4) is for minimizing the spherical aberration caused by the rear group G₃₂ in the third lens group under the above condition of Equation (3). If the condition of Equation (4) is not satisfied, the load increases on the compensation of spherical aberration in the front group G₃₁ in the third lens group, which undesirably makes the compensation of spherical aberration difficult.

Further, the following condition is more preferably satisfied.

    -0.47<f.sub.2 /f.sub.1 <-0.37                              (5)

The condition of Equation (5) is a condition for well compensating the aberration of off-axial rays by defining a ratio of focal length f₂ of the second lens group G₂ to focal length f₁ of the first lens group G₁.

Above the upper limit of the condition of Equation (5), the gap increases between the first lens group G₁ and the second lens group G₂, so that the off-axial rays enter the second lens group G₂ at positions near the optical axis. This undesirably makes it difficult for the second lens group G₂ to compensate the astigmatism caused by the first lens group G₁. Conversely, below the lower limit of the condition of Equation (5), the gap decreases between the first lens group G₁ and the second lens group G₂, so that the Rand rays enter the second lens group G₂ at positions far from the optical axis. This increases the outer diameter of the second lens group G₂ as the focusing group. Then, the load unpreferably increases on the drive motor for focusing group in autofocus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing to show a lens arrangement in Embodiment 1 according to the present invention;

FIG. 2 is a drawing to show a lens arrangement in Embodiment 2 according to the present invention;

FIG. 3 is a drawing to show a lens arrangement in Embodiment 3 according to the present invention;

FIG. 4 is a drawing to show a lens arrangement in Embodiment 4 according to the present invention; and

FIG. 5 is a drawing to show a lens arrangement in Embodiment 5 according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments according to the present invention will be next described in detail with reference to the accompanying drawings.

FIG. 1 to FIG. 5 are drawings respectively to show a lens arrangement in each of Embodiments 1-5 according to the present invention. In each embodiment as shown in FIG. 1 to FIG. 5, the lens is constituted by a first lens group G₁ of positive refracting power, a second lens group G₂ of negative refracting power and a third lens group G₃ of positive refracting power arranged in the named order from the object side, in which the first lens group G₁ and the second lens group G₂ form a substantially afocal system and, in focusing from the infinity to a near range object, the second lens group G₂ moves toward the image.

In each embodiment, an aperture stop S is disposed between the second lens group G₂ and the third lens group G₃.

Embodiment 1

FIG. 1 is a drawing to show a lens arrangement in Embodiment 1 according to the present invention. As shown in FIG. 1, Embodiment 1 of telephoto lens is constructed in the following order from the object side, of a first lens group G₁ composed of a front group G₁₁ including two positive double convex lenses, a negative double concave lens, and a positive meniscus lens with a convex surface on the object side, and a rear Group G₁₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side; a second lens Group G₂ composed of a compound lens of a positive meniscus lens with a concave surface on the object side and a negative double concave lens, and a negative double concave lens; an aperture stop S; a third lens Group G₃ composed of a front Group G₃₁ which is a compound lens of a positive double convex lens with a stronger convex surface on the image side and a negative meniscus lens with a convex surface on the image side, and a rear Group G₃₂ of a positive double convex lens; and a stationary stop A.

The following table lists specifications in Embodiment 1 of the present invention. In the table of specifications in each embodiment, each number in the left end column represents an order counted from the object side, r a radius of curvature of lens surface, d a distance between lens surfaces, n and ν an index of refraction and an Abbe's number for d line (λ=587.6 nm), respectively, f a focal length, F_(no) an f-number, β an image magnification, and D0 a distance between an object and the first lens surface.

                  TABLE 1                                                          ______________________________________                                         (Specifications for Embodiment 1)                                              f = 195.00 mm                                                                  F.sub.no = 2                                                                   r            d              ν   n                                           ______________________________________                                          1    148.750    11.50          82.6 1.49782                                    2    -1205.785  0.30                                                           3    142.017    14.50          82.6 1.49782                                    4    -320.577   2.00                                                           5    -307.636   3.70           35.2 1.74950                                    6    280.469    1.00                                                           7    136.923    8.00           82.6 1.49782                                    8    348.636    30.90                                                          9    121.116    3.90           60.0 1.64000                                   10    41.001     15.00          67.9 1.59319                                   11    178.431    (d11)                                                         12    -1353.905  6.00           23.8 1.84666                                   13    -105.258   3.00           64.1 1.51680                                   14    76.392     7.10                                                          15    -104.451   3.50           65.7 1.46450                                   16    61.871     (d16)                                                         17    1692.861   7.50           53.7 1.69350                                   18    -47.031    2.50           28.2 1.74000                                   19    -306.591   0.30                                                          20    129.809    6.00           49.4 1.77279                                   21    -165.858   11.00                                                         22    ∞    (Bf)                                                                f = 195.0000                                                                              β = -0.1201                                              D0    ∞    1765.5380                                                     d11   5.9982     18.7714                                                       d16   22.7762    10.0030                                                       Bf    67.9864    67.9864                                                       ______________________________________                                    

There are listed in the following for Embodiment 1 a focal length of each lens group, an effective aperture φ of object-side lens surface of the positive lens element closest to the object in the first lens group G₁, a back focus Bf₁₁ of the front group G₁₁ in the first lens group G₁, and an air gap D₁ between the front group G₃₁ and the rear group G₃₂ in the third lens group.

f₁ =144.000

f₂ =-60.000

f₃ =81.250

f₃₁ =545.252

φ=97.500

Bf₁₁ =137.408

D₁ =30.901

Next listed are values for the aforementioned conditions in Embodiment 1.

(1)φ/f₁ =0.677

(2)D₁ /Bf₁₁ =0.225

(3)f₃ /f₃₁ =0.149

(4)q₃₂ =0.122

(5)f₂ /f₁ =-0.417

Embodiment 2

FIG. 2 is a drawing to show a lens arrangement in Embodiment 2 according to the present invention. As shown in FIG. 2, Embodiment 2 of telephoto lens is constructed in the following order from the object side, of a first lens group G₁ composed of a front group G₁₁ including two positive double convex lenses and a negative double concave lens, and a rear group G₁₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side; a second lens group G₂ composed of a compound lens of a positive meniscus lens with a concave surface on the object side and a negative double concave lens, and a negative double concave lens; an aperture stop S; a third lens group G₃ composed of a front group G₃₁ which is a compound lens of a positive meniscus lens with a convex surface on the image side and a negative meniscus lens with a convex surface on the image side, and a rear group G₃₂ of a positive double convex lens; and a stationary stop A.

The following table lists specifications in Embodiment 2 of the present invention. In the table of specifications in each embodiment, each number in the left end column represents an order counted from the object side, r a radius of curvature of lens surface, d a distance between lens surfaces, n and ν an index of refraction and an Abbe's number for d line (λ=587.6 nm), respectively, f a focal length, F_(no) an f-number, β an image magnification, and D0 a distance between an object and the first lens surface.

                  TABLE 2                                                          ______________________________________                                         (Specifications for Embodiment 2)                                              f = 195.00 mm                                                                  F.sub.no = 2                                                                   r            d              ν   n                                           ______________________________________                                          1    128.016    15.00          82.6 1.49782                                    2    -566.110   0.30                                                           3    122.059    14.50          82.6 1.49782                                    4    -331.164   1.00                                                           5    -306.978   3.70           35.2 1.74950                                    6    261.822    38.33                                                          7    96.244     3.90           60.0 1.64000                                    8    38.562     15.00          67.9 1.59319                                    9    159.310    (d9)                                                          10    -563.672   6.00           23.8 1.84666                                   11    -93.627    3.00           64.1 1.51680                                   12    84.852     6.40                                                          13    -110.869   3.50           65.7 1.46450                                   14    58.532     (d14)                                                         15    -6430.227  7.50           53.7 1.69350                                   16    -45.538    2.50           28.2 1.74000                                   17    -288.190   0.30                                                          18    123.848    6.00           49.4 1.77279                                   19    -163.413   11.00                                                         20    ∞    (Bf)                                                                f = 195.0000                                                                              β = -0.1198                                              D0    ∞    1765.2004                                                     d9    5.9970     18.7416                                                       d14   22.6002    9.8556                                                        Bf    68.2697    67.2697                                                       ______________________________________                                    

There are listed in the following for Embodiment 2 a focal length of each lens group, an effective aperture φ of object-side lens surface of the positive lens element closest to the object in the first lens group G₁, a back focus Bf₁₁ of the front group G₁₁ in the first lens group G₁, and an air gap D₁ between the front group G₃₁ and the rear group G₃₂ in the third lens group.

f₁ =144.000

f₂ =-60.000

f₃ =81.250

f₃₁ =692.673

φ=97.500

Bf₁₁ =156.742

D₁ =38.333

Next listed are values for the aforementioned conditions in Embodiment 2.

(1)φ/f₁ =0.677

(2)D₁ /Bf₁₁ =0.245

(3)f₃ /f₃₁ =0.117

(4)q₃₂ =0.138

(5)f₂ /f₁ =-0.417

Embodiment 3

FIG. 3 is a drawing to show a lens arrangement in Embodiment 3 according to the present invention. As shown in FIG. 3, Embodiment 3 of telephoto lens is constructed in the following order from the object side, of a first lens group G₁ composed of a front group G₁₁ including two positive double convex lenses and a negative double concave lens, and a rear group G₁₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side; a second lens group G₂ composed of a compound lens of a positive meniscus lens with a concave surface on the object side and a negative double concave lens, and a negative double concave lens; an aperture stop S; a third lens group G₃ composed of a front group G₃₁ composed of a positive double convex lens with a stronger convex surface on the image side and a negative meniscus lens with a convex surface on the image side, and a rear group G₃₂ of a positive double convex lens; and a stationary stop A.

The following table lists specifications in Embodiment 3 of the present invention. In the table of specifications in each embodiment, each number in the left end column represents an order counted from the object side, r a radius of curvature of lens surface, d a distance between lens surfaces, n and ν an index of refraction and an Abbe's number for d line (λ=587.6 nm), respectively, f a focal length, F_(no) an f-number, β an image magnification, and D0 a distance between an object and the first lens surface.

                  TABLE 3                                                          ______________________________________                                         (Specifications for Embodiment 3)                                              f = 195.00 mm                                                                  F.sub.no = 2                                                                   r            d              ν   n                                           ______________________________________                                          1    122.647    15.00          82.6 1.49782                                    2    -677.504   0.50                                                           3    127.484    14.00          82.6 1.49782                                    4    -426.939   2.00                                                           5    -353.235   5.00           35.2 1.74950                                    6    257.054    29.10                                                          7    87.534     4.00           55.6 1.69680                                    8    42.733     13.00          67.9 1.59319                                    9    135.210    (d9)                                                          10    -866.539   6.00           23.8 1.84666                                   11    -108.725   3.00           64.1 1.51680                                   12    80.309     7.10                                                          13    -114.889   3.50           65.7 1.46450                                   14    62.031     (d14)                                                         15    289.420    7.20           53.7 1.69350                                   16    -54.834    1.20                                                          17    -53.390    2.40           27.6 1.74077                                   18    -545.430   0.50                                                          19    119.482    6.00           49.4 1.77279                                   20    -190.308   5.00                                                          21    ∞    (Bf)                                                                f = 195.0000                                                                              β = -0.1211                                              D0    ∞    1764.9781                                                     d9    16.5418    30.9892                                                       d14   25.8092    11.3618                                                       Bf    68.1697    68.1697                                                       ______________________________________                                    

There are listed in the following for Embodiment 3 a focal length of each lens group, an effective aperture φ of object-side lens surface of the positive lens element closest to the object in the first lens group G₁, a back focus Bf₁₁ of the front group G₁₁ in the first lens group G₁, and an air gap D₁ between the front group G₃₁ and the rear group G₃₂ in the third lens group.

f₁ =152.500

f₂ =-61.000

f₃ =78.000

f₃₁ =364.073

φ=97.500

Bf₁₁ =158.546

D₁ =29.099

Next listed are values for the aforementioned conditions in Embodiment 3.

(1)φ/f₁ =0.639

(2)D₁ /Bf₁₁ =0.184

(3)f₃ /f₃₁ =0.214

(4)q₃₂ =0.229

(5)f₂ /f₁ =-0.400

Embodiment 4

FIG. 4 is a drawing to show a lens arrangement in Embodiment 4 according to the present invention. As shown in FIG. 4, Embodiment 4 of telephoto lens is constructed in the following order from the object side, of a first lens group G₁ composed of a front group G₁₁ including two positive double convex lenses, a negative double concave lens, and a positive meniscus lens with a convex surface on the object side, and a rear group G₁₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side; a second lens group G₂ composed of a compound lens of a positive meniscus lens with a concave surface on the object side and a negative double concave lens, and a negative double concave lens; an aperture stop S; a third lens group G₃ composed of a front group G₃₁ composed of a positive double convex lens with a stronger convex surface on the image side and a negative meniscus lens with a convex surface on the image side, and a rear group G₃₂ of a positive double convex lens; and a stationary stop A.

The following table lists specifications in Embodiment 4 of the present invention. In the table of specifications in each embodiment, each number in the left end column represents an order counted from the object side, r a radius of curvature of lens surface, d a distance between lens surfaces, n and ν an index of refraction and an Abbe's number for d line (λ=587.6 nm), respectively, f a focal length, F_(no) an f-number, β an image magnification, and D0 a distance between an object and the first lens surface.

                  TABLE 4                                                          ______________________________________                                         (Specifications for Embodiment 4)                                              f = 198.00 mm                                                                  F.sub.no = 2                                                                   r            d              ν   n                                           ______________________________________                                          1    176.600    12.00          82.5 1.49782                                    2    -980.896   0.30                                                           3    117.075    17.00          82.5 1.49782                                    4    -291.364   1.80                                                           5    -284.799   3.50           35.2 1.74950                                    6    240.757    1.00                                                           7    157.380    8.00           82.5 1.49782                                    8    451.625    22.14                                                          9    127.025    4.00           60.0 1.64000                                   10    50.405     16.00          67.9 1.59319                                   11    296.235    (d11)                                                         12    -10000.000 6.00           23.0 1.86074                                   13    -137.877   3.00           70.4 1.48749                                   14    68.971     8.00                                                          15    -102.684   3.50           65.8 1.46450                                   16    65.522     (d16)                                                         17    244.766    8.50           53.7 1.69350                                   18    -60.716    1.00                                                          19    -60.242    3.00           28.3 1.72825                                   20    -7493.004  0.50                                                          21    200.911    6.50           49.5 1.77279                                   22    -147.244   17.00                                                         23    ∞    (Bf)                                                                f = 198.0000                                                                              β = -0.1207                                              D0    ∞    1763.7176                                                     d11   4.9176     15.7144                                                       d16   20.6778    9.8810                                                        Bf    67.9434    67.9434                                                       ______________________________________                                    

There are listed in the following for Embodiment 4 a focal length of each lens group, an effective aperture φ of object-side lens surface of the positive lens element closest to the object in the first lens Group G₁, a back focus Bf₁₁ of the front Group G₁₁ in the first lens Group G₁, and an air gap D₁ between the front Group G₃₁ and the rear Group G₃₂ in the third lens Group.

f₁ =133.100

f₂ =-60.500

f₃ =90.000

f₃₁ =409.757

φ=99.000

Bf₁₁ =141.509

D₁ =22.144

Next listed are values for the aforementioned conditions in Embodiment 4.

(1)φ/f₁ =0.744

(2)D₁ /Bf₁₁ =0.157

(3)f₃ /f₃₁ =0.220

(4)q₃₂ =-0.154

(5)f₂ /f₁ =-0.455

Embodiment 5

FIG. 5 is a drawing to show a lens arrangement in Embodiment 5 according to the present invention. As shown in FIG. 5, Embodiment 5 of telephoto lens is constructed in the following order from the object side, of a first lens Group G₁ composed of a front Group G₁₁ including two positive double convex lenses and a negative double concave lens, and a rear Group G₁₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive meniscus lens with a convex surface on the object side; a second lens group G₂ composed of a compound lens of a positive meniscus lens with a concave surface on the object side and a negative double concave lens, and a negative double concave lens; an aperture stop S; a third lens group G₃ composed of a front group G₃₁ composed of a positive double convex lens with a stronger convex surface on the image side and a negative meniscus lens with a convex surface on the image side, and a rear group G₃₂ which is a compound lens of a negative meniscus lens with a convex surface on the object side and a positive double convex lens; and a stationary stop A.

The following table lists specifications in Embodiment 5 of the present invention. In the table of specifications in each embodiment, each number in the left end column represents an order counted from the object side, r a radius of curvature of lens surface, d a distance between lens surfaces, n and ν an index of refraction and an Abbe's number for d line (λ=587.6 nm), respectively, f a focal length, F_(no) an f-number, β an image magnification, and D0 a distance between an object and the first lens surface.

                  TABLE 5                                                          ______________________________________                                         (Specifications for Embodiment 5)                                              f = 195.00 mm                                                                  F.sub.no = 2                                                                   r           d              Abbe    n                                           ______________________________________                                          1   135.402    14.00          82.5  1.49782                                    2   -758.266   0.30                                                            3   120.706    15.00          82.5  1.49782                                    4   -332.284   1.30                                                            5   -315.793   3.80           35.2  1.74950                                    6   314.945    37.732                                                          7   119.204    3.80           60.0  1.64000                                    8   40.575     14.00          67.9  1.59319                                    9   188.007    (d9)                                                           10   -1133.841  5.20           23.0  1.86074                                   11   -99.640    2.80           64.1  1.51680                                   12   86.754     6.50                                                           13   -122.462   3.30           45.9  1.54814                                   14   61.980     (d14)                                                          15   264.265    7.00           53.7  1.69350                                   16   -58.429    1.00                                                           17   -57.994    2.50           28.2  1.74000                                   18   -294.241   4.20                                                           19   156.674    2.00           35.5  1.59507                                   20   61.130     6.50           47.5  1.78797                                   21   -301.397   3.00                                                           22   ∞    (Bf)                                                                f = 195.0000                                                                              β = -0.1230                                               D0   ∞    1761.1081                                                      d9   14.4358    29.0347                                                        d14  23.0099    8.4109                                                         Bf   64.7804    64.7804                                                        ______________________________________                                    

There are listed in the following for Embodiment 5 a focal length of each lens group, an effective aperture φ of object-side lens surface of the positive lens element closest to the object in the first lens group G₁, a back focus Bf₁₁ of the front group G₁₁ in the first lens group G₁, and an air gap D₁ between the front group G₃₁ and the rear group G₃₂ in the third lens group.

f₁ =153.400

f₂ =-59.000

f₃ =75.000

f₃₁ =228.789

φ=97.500

Bf₁₁ =154.506

D₁ =37.732

Next listed are values for the aforementioned conditions in Embodiment 5.

(1)φ/f₁ =0.636

(2)D₁ /Bf₁₁ =0.244

(3)f₃ /f₃₁ =0.328

(4)q₃₂ =0.316

(5)f₂ /f₁ =-0.385

The image can be deflected either by making the third lens group G₃ offset from the optical axis or by making either the front group G₃₁ or the rear group G₃₂ in the third lens group offset from the optical axis. 

What is claimed is:
 1. A high-aperture-ratio inner-focus telephoto lens comprising a first lens group of positive refracting power, a second lens group of negative refracting power and a third lens group of positive refracting power arranged in the named order from the object side;wherein said first lens group and said second lens group form a substantially afocal system; wherein focusing is effected by moving said second lens group along the optical axis; wherein said first lens group has a front group of positive refracting power and a rear group of weak positive refracting power arranged in the named order from the object side; wherein said third lens group comprises a front group of weak positive refracting power and a rear group of positive refracting power arranged in the named order from the object side; and wherein the following conditions are satisfied:
 0. 60<Φ/f₁ <0.77

    0.10<D.sub.1 /Bf.sub.11 <0.26

where Φ is an effective diameter of an object-side lens surface of a positive lens element closest to an object in said first lens group, f₁ is a focal length of said first lens group, Bf₁₁ is a back focus of said front group in said first lens group and D₁ is an air gap between said front group and said rear group in said first lens group.
 2. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein the following condition is satisfied:

    0.10<f.sub.3 /f.sub.31 <0.35

where f₃ is a focal length of said third lens group and f₃₁ is a focal length of said front group in said third lens group.
 3. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said front group in said third lens group comprises at least one positive lens element and a negative lens element.
 4. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said rear group in said third lens group comprises a positive lens element, or a positive lens element and a negative lens element.
 5. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said rear group in said third lens group is arranged to satisfy the following condition:

    -0.17<q.sub.32 <0.35

where q₃₂ is a dimensional factor of said rear group in said third lens group and q₃₂ =(R₂ +R₁)/(R₂ -R₁) where R₁ : a radius of curvature of a surface closest to an object in said rear group in said third lens group; R₂ : a radius of curvature of a surface closest to an image in said rear group in said third lens group.
 6. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said rear group in said third lens group is arranged movable across the optical axis to deflect an image thereby.
 7. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said front group in said third lens group is arranged movable across the opticalaxis to deflect an image thereby.
 8. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein the following condition is satisfied:

    -0.47<f.sub.2 /f.sub.1 <-0.37

where f₁ is a focal length of said first lens group and f₂ is a focal length of said second lens group.
 9. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said rear group in said first lens group has a negative lens element of meniscus form with a convex surface on the object side and a positive lens element of meniscus form with a convex surface on the object side arranged in the named order from the object side.
 10. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein an aperture stop is provided in an optical path between said second lens group G₂ and said third lens group.
 11. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein a stationary stop is provided in an optical path on the image side of said third lens group.
 12. A high-aperture-ratio inner-focus telephoto lens according to claim 1, wherein said third lens group is arranged movable across the optical axis to deflect the image thereby.
 13. A high-aperture-ratio inner-focus telephoto lens comprising a first lens group of positive refracting power, a second lens group of negative refracting power and a third lens group of positive refracting power arranged in the named order from the object side;wherein said first lens group and said second lens group form a substantially afocal system; wherein focusing is effected by moving said second lens group along the optical axis; wherein said first lens group has a front group of positive refracting power and a rear group of weak positive refracting power arranged in the named order from the object side; and wherein said third lens group is arranged movable across the optical axis to deflect the image thereby. 